In several applications of the use of slide projectors, such as during operational training of workers or in advertising, additional capabilities are desired which are not provided by conventional slide projectors. These capabilities or additional functions include fading or blending and mixing at several blending speeds, the projection of indicators (i.e., pointers), the projection of graphic arts upon another image, comparison projections, etc. Manual, mechanical or electrical controls can be provided for slide projectors in order to realize the special or supplementary functions.
A projection mechanism using two cartridge-or magazine-type projectors having a driven motor, a frame-change mechanism and a common control member is known, where, upon the switching off of one projector, th other one is set in motion (German patent application No. 1,572,714). In this projection mechanism, the control part contains a release member having electrical follow-on circuits, by means of which the bulbs of both projectors are alternately capable of being switched in and out. Also, when a bulb is switched out, a frame change is effected in the associated projector. Delay means are further associated with the follow-on circuits. By means of the delay means, the commencement of an image change is delayed at least by the bulb persistence following switch-off. A significant disadvantage of this mechanism is that fading, or blending and mixing is only possible during the forward run of the slide magazines or cartridges, so that the sequence of slides shown cannot be reversed.
In another control mechanism, it is possible to carry out fading or blending and mixing projections during forward and reverse motion (German Pat. No. 2,145,100). Here, the fading or mixing and blending proces is temporally pre- and post-coupled during respective forward and reverse motions to the cartridge or magazine transport. In this manner, the alternating switching of the projectors during forward and reverse motions of the magazines or cartridges is accomplished by respective first and second flip-flops with correspondingly associated switches.
The output signals of the first flip-flop, together with signals from a blending and mixing circuit control an AND gate. The cartridge or magazine transport circuits of the projectors are controlled via AND gates for respective forward and reverse motions. It is a disadvantage of these control circuits that they can basically only realize a mixing and blending projection.
It has also been suggested that digital control and permanent memory systems be utilized for the control and programming of large multi-projection systems (see E. Stechmesser: "Steuerung und Programmierung von grossen Multiprojektionsanlagen": Photo-technik und -wirschaft No. 1, 1974, s. 11). Large systems of this type are not, however, presently in use.